Process for inhibiting popcorn polymer formation

ABSTRACT

PROCESS OF RETARDING THE FORMATION OF POPCORN POLYMERS BY USE OF AN AROMATIC AMINE OXIDE OF STRUCTURE   (O=N(-R1)2-),X-BENZENE   WHERE R1 IS LOWER ALKYL, HYDROXY SUBSTITUTED LOWER ALKYL, OR A GROUP OF STRUCTURE   (1-(HO-),R2,R3-PHENYL)-CH2-   WHERE R2 IS HYDROGEN OR LOWER ALKYL, AND R3 IS A TERTIARY ALKYL GROUP, X IS HYDROGEN, HALOGEN, NITRO, OR LOWER ALKYL, AND WITH THE PROVISO THAT THE TOTAL NUMBER OF CARBON ATOMS IN THE R1 GROUPS BE AT LEAST FOUR.

United "States Patent US. Cl. 26094.7 N 7 Claims ABSTRACT OF THEDISCLOSURE Process of retarding the formation of popcorn polymers by useof an aromatic amine oxide of structure where R is lower alkyl, hydroxysubstituted lower alkyl, or a group of structure where R is hydrogen orlower alkyl, and R is a tertiary alkyl group, X is hydrogen, halogen,nitro, or lower alkyl, and with the proviso that the total number ofcarbon atoms in the R groups be at least four.

This application is a divisional of SN. 681,093, filed Nov. 7, 1967.

In the preparation of synthetic rubber from such intermediates asstyrene and butadiene (e.g., SBR elastomers) undesirable spontaneouspolymerization often occurs in the recovery systems for the olefinicallyunsaturated monomers. Polymerization may occur to form either a clear,solid, aromatic solvent soluble polymer or to form an entirely diiferentcross-linked insoluble polymer, known, because of its appearance, aspopcorn polymer. While both types of this self-polymerization areobjectionable, the popcorn polymer, which is predominantly formed, isparticularly undesirable because it is selfpropagating in the presenceof the monomer vapor or liquid and not only rapidly fouls the equipment,but is very difiicult to remove and control. When such popcorn polymersdo form it frequently becomes necessary to disassemble the equipment andmechanically remove the accumulations of unwanted polymer.

Much work has been done to find suitable inhibitors to prevent popcornpolymer formation. Nitrites and nitroso compounds have been found to beeffective as have N0 N 0 certain phenolic compounds, sulfur and somearomatic amines. However, each of these agents leaves something to bedesired for commercially effective use. Some of the agents are difficultto handle; others introduce Patented Jan. 4, 1972 colored impuritiesinto the recovered olefins; some, although having the desiredproperties, are too expensive to be of commercial utility.

Because of the uniqueness of the popcorn polymer, and the manner inwhich it is formed, there is no correlation between popcorn polymerinhibition and monomer stabilization which involves the prevention ofpremature polymerization of olefins during shipping or storage. Thus,for example, it is disclosed in US. 2,667,442 that amine oxides such asdimethylaniline oxide are useful to prevent the premature polymerizationof styrene. HOW- ever, dimethylaniline oxide is not effective as apopcorn polymer inhibitor. This is consistent with disclosures ofvarious other agents reported to be stabilizers for Olefinic monomerswhich are not effective as popcorn polymer inhibitors.

It has now been found by means of this invention that effectiveinhibition of popcorn polymer formation can be achieved by the use of acompound of structure where R is lower alkyl, hydroxy substituted loweralkyl, or a group of structure where R is hydrogen or lower alkyl, and Ris a tertiary alkyl group, X is hydrogen, halogen, nitro, or loweralkyl, and with the proviso that the total number of carbon atoms in theR groups be at least four.

In the above structural formula lower alkyl refers to those alkyl groupscontaining 1 to 4 carbon atoms and the term tertiary alkyl group refersto groups such as butyl, amyl, hexyl, octyl, and like groups which areattached to the aromatic ring by means of a tertiary carbon atom. Asalso indicated, the total number of carbon atoms in the R groups mustexceed four and this will exclude from the invention compounds such asdimethylaniline-N- oxide and methylethylaniline-N-oxide. Specificcompounds falling within the above description of operable compoundsare:

diethylaniline-N-oxide,

o-nitro-diethylaniline-N-oxide,

p-nitrodiethylaniline-N-oxide,

o-chlorodiethylaniline-N-oxide,

p-bromodipropylaniline-N-oxide,

o-iododibutylaniline-N-oxide,

o-methyldiethylaniline-N-oxide,

2,2- (phenylimino) diethanol-N-oxide,

4,4'- (phenylimino dibutanol-N-oxide,

N-rnethyl-N- (2-hydroxy-3 -t-butyl-5-methylbenzyl) aniline-N-oxide,

N-methyl-N- Z-hydroxy-S-t-amylbenzyl) aniline-N-oxide,

and the like.

All of the above compounds except diethylaniline-N-oxide are novelcompounds and are an embodiment of this invention. Thus the genericconfiguration of these novel compounds is as given above by thestructural formula except that X in the structure is other thanhydrogen. These novel compounds are readily made from the correspondingamine by oxidation with hydrogen peroxide as shown in the subsequentexamples.

As already indicated, in the manufacture of synthetic rubber the problemof popcorn polymer formation is pecular to the monomer recovery systemwhere the monomers are recovered by distillation. The conventionalclosed system for the emulsion polymerization of butadiene with styrenecomprises a conventional reaction vessel equipped with a stirringmechanism and necessary heating or cooling means in which the monomersare caused to polymerize. After a suitable degree of polymerization isachieved, the polymerization reaction is stopped by the addition of asuitable stopping agent. The resulting polymer latex is then allowed toflow into a flash tank which is at or slightly above atmosphericpressure and at which time most of the residual butadiene is removedfrom the latex. The gaseous butadiene is then removed from the flashtank and liquified for reuse. The butadiene degassed latex is thenallowed to flow into a conventional vacuum flash tank where furtherbutadiene and other dissolved gaseous materials are removed. The vacuumflash tank is maintained at a temperature of about 100 F. It is in thisvacuum flash tank that the most ideal conditions for popcorn polymerformation exist because the tank is at the proper temperature; theatmosphere above the level of latex contained in the tank contains about2% or less of butadiene; and a certain amount of catalyst has vaporizedand collected on the inner exposed metal surfaces of the tank above thelevel of the latex. These conditions will initiate popcorn polymer. Thepopcorn polymer will continue to grow as long at it is fed by a newsupply of latex containing a small proportion of butadiene and otherpolymerizable monomer, such as styrene. The pipe lines leading to andfrom this vacuum flash tank are also ideal areas for popcorn polymerformation.

The latex is then pumped from the vacuum flash tank to a conventionalstyrene stripping column where the latex is passed counter-current to arising stream of steam causing the styrene to be removed from the latexwhere it is then recirculated in a conventional manner to the reactionvessel for polymerization with butadiene. In the styrene strippingcolumn popcorn polymer formation also tends to develop unless someprecautions are taken to prevent its development.

Preferably, the inhibitor is added to a flash tank used in the recoveryprocess. However, the inhibitor may be introduced to the monomer at anystage in the manufacture of synthetic rubber, as for example during themanufacture, handling, storage, etc. of the intermediates. For example,the inhibitor vapor can be introduced as the gaseous monomer is beingpassed through pipes, it can be mixed with the monomer in process tanks,or, as indicated, it can be introduced during the fractionaldistillation of materials in the recovery systems of the rubbermanufacturing process. In the preferred technique it is considered bestto feed the monomer into a flash tank or fractional distillation columnof conventional type. The monomer is subject to fractional distillationusing conventional reboiling at the bottom of the column and withdrawalof overhead material at the top, condensing the overhead material andreturning a portion of it to the top of the column as reflux. Theinhibiting vapor or solution is continuously fed, preferably by sprayingits solution in water or monomer into the upper portion of the columnthrough which it descends. In other techniques the inhibitor can beintroduced to one or more of the monomers in any phase wherein themonomer is being circulated in the process.

The concentration at which the inhibitor is used will usually range from0.001 to about 5.0 percent by weight of the total monomers (i.e. about10 to 50,000 parts per million parts of monomer). At concentration belowthis value the inhibiting effects are to small to be of significantvalue. On the other hand, greater amounts may be used, say up to 20%,but such large amounts are not required and are simply wasteful ofinhibitor.

It is to be understood that the amine oxide inhibitors may be usedgenerally to prevent popcorn polymers in the preparation of polymers andcopolymers such as those obtained from ethylenically unsaturatedmonomers, For homopolymers, the unsaturated monomer will be a conjugateddiolefin. The useful conjugated diolefins are exemplified by butadienesuch as butadiene-1,3, isoprene, cyanobutadiene-1,3, chloroprene,Z-phenylbutadiene, 2,3-dimethylbutadiene-l,3, and the like. Thecopolymerizable monomer used in copolyrner formation and which willnormally comprise up to about 70% of the mixture will be a mono-olefincontaining a single CH =CH- group having at least one of the freevalence bonds attached to an electronegative group. Such olefins includearomatic olefins such as styrene, vinylnaphthaline, a-methylstyrene,pchlorostyrene, etc.; the carboxy containing monomers and thecorresponding esters, nitriles, and amides such as acrylic acid,methacrylic acid, methyl methacrylate, acrylo nitrile, methacrylamide,and the like. Preferably, this invention will be used in recovering themonomers used to make any butadiene-based polymer latex.

In order to illustrate the effectiveness of the invention, the followingexamples are given:

EXAMPLES (A) Preparation of active compounds (1) Preparation ofo-nitrodiethylaniline-N-oxide-A solution of 7.8 g. (0.04 mole) ofo-nitrodiethylaniline in 40 ml. of 98% formic acid was heated to 55-60"C. and then 19.1 g. (0.186 mole) of 35% hydrogen peroxide was addedropwise during stirring. The resulting reaction mixture was heated at5560 C. with stirring for 2 hours. The cooled reaction mixture wasdiluted with 50 ml. of water and then it was neutralized withconcentrated ammonium hydroxide. This solution was extracted once withether and the ether extract was discarded. The product was extractedwith seven 30 ml. portions of chloroform. Evaporation of the chloroformsolution gave 3.8 g. (45% conversion) of crude product, M.P. -90" C.

After recrystallization from acetone-hexane the melting point of the tansolid obtained was 93 C.

Analysis (percent): N=l3.18. Calcd (percent): N=13.35. The structure ofthe product was confirmed by infrared absorption analysis.

(2) Preparation of p chloro N,N-diethylaniline-N oxide(monohydrate).Asolution of 11.0 g. (0.06 mole) of p-chlorodiethylaniline in 60 m1. of98% formic acid was heated to 55-60 C. and then 28.7 g. (0.294 mole) of35% hydrogen peroxide was added dropwise during stirring. The reactionmixture was heated at 5560 C. with stirring for 2 hrs. The cooledreaction mixture was diluted with 70 ml. of water and neutralized withconcentrated ammonium hydroxide. This was extracted with seven 30 ml.portions of chloroform. Evaporation of the solvent gave 15 g. of aresidue which was recrystallized from acetone-hexane to give 8.0 g.(61.3% conversion) of a white solid having a melting point of 79-81 C. Asmall portion of this solid recrystallized from acetone had a meltingpoint of 8284 C.

Analysis (percent): C, 55.26; H, 7.44; N, 6.52. Calculated (percent): C,55.2; H, 7.34; N, 6.44. The structure of the product was confirmed byinfrared absorption analysis.

(3) N methyl N(2 hydroxy 3-t-butyl-5-methylbenzyl) aniline-N-oxide.-Thepresent amine was prepared by the following sequence of reactionsdescribed in US. 3,219,700:

t-C4Ho 2 (CH3)ZNH(25%) t-C-rHo CH2N(CH )g t-C H9 CHzSCN(CH3)2 (I) est-CH (II) (II) CH3NHC5H5 NaOH OH CH;

t-C4H9 CHgN Ce s CH (III) (CH )gI| |TCSNa S M.P.=6769 C.

To a solution of 14.1 g. (0.05 mole) of (III) in 50 ml. of boilingmethanol, 9.7 g. (0.1 mole) of 35% H 0 was added. The reaction mixturewas refluxed 3 days and at the end of this period no peroxide remained,(iodometric titration). The solvent was removed in a flash evaporaterat 55-60 C. under reduced pressure. The residue, a dark brown viscousoil weighed 14.7 g. (98.3% yield).

Analysis (percent): C, 77.56; H, 8.57; N, 4.20. Calculated (percent): C,76.4; H, 8.36; N, 4.68. The structure of the product was confirmed byinfrared absorption analysis.

(4) N ethyl N-(2-hydroxy-5-t-amylbenzyl)aniline- N-oxide (dihydrate).Theparent amine was prepared by the following sequence of reaction:

(an oil which crystallizes on prolonged standing) (I) A solution of 29.7g. (.01 mole) of (II) in 4.0 ml. methanol was treated with 19.4 g. (0.2mole) 35% H 0 The reaction mixture was refluxed 5 /2 hrs. Titrationshowed no peroxide to be present. The solvent was removed in a flashevaporator at 55-60 C. under reduced pressure to give 33.0 g. (94.5%yield) of a dark brown viscous oil.

Analysis (percent): C, 69.74; H, 8.77; N, 3.94. Calculated (percent): C,69.0; H, 8.91; N, 4.03. The structure of this product was confirmed byinfrared absorption analysis.

(B) Evaluation techniques Procedures used for evaluating a compound asan inhibitor for popcorn polymer in the liquid phase involved flushing 7oz. beverage bottles with nitrogen, then charging each bottle with ml.of inhibitor-free styrene, 0.5

g. of popcorn polymer seed and the material to be tested as the popcorninhibitor. The seed, from plant flash tanks, was activated just beforeusage by overnight exposure to a watt incandescent lamp. The bottleswere capped and 1 ml. of liquid butadiene was injected into each bottlewith a hypodermic syringe through the self-sealing NBR synthetic rubbercap liner. The bottles were then placed in a constant temperature ovenat 60 C. and were inspected periodically for the appearance of popcornpolymer. After popcorn growth started in each bottle it often proceededrapidly, filling most of the free space in the bottle with whiteinsoluble polymer having the appearance of popcorn. The time requiredfor initiation of popcorn growth and the rate of growth depend somewhaton the activity of the seed used. Such seed usually becomes less activeon standing. The inhibitors of this invention were found to affect boththe time required for the initiation of popcorn growth and the rate ofgrowth. In some cases the growth proceeded to a low conversion andstopped, the inhibitor preventing complete conversion to popcornpolymer. In the presentation of data, the time for initiation of popcornpolymer growth is given.

The following Table I indicates the results of the test technique.

TABLE I Evaluation of aromatic amine oxides as popcorn polymerinhibitors Compound tested at 0.1% concentration:

Time in days for start of popcorn polymer growth at 60 C.

It will be understood that the above description and examples are merelyillustrative and that numerous changes and variations may be madewithout departing from the spirit or scope of the invention.

We claim:

1. The process of retarding the formation of popcorn polymers in monomerrecovery systems in the preparation of synthetic rubber from conjugateddiolefin-containing monomer systems which comprises contacting saidmonomer with an inhibiting amount of a compound having the structure.

wherein R is lower alkyl, hydroxy substituted lower alkyl, or a group ofstructure where R is hydrogen or lower alkyl, R is a tertiary alkylgroup, X is hydrogen, halogen, nitro, or lower alkyl, and

8 With the proviso that the total number of carbon atoms in ReferencesCited the R groups be at least four.

2. The process of claim 1, Where the active agent is UNITED STATESPATENTS diethylaniline oxide. 3,518,320 6/1970 Albert 260-6665 3. Theprocess of claim 1, where the active agent is 5 3 524,394 3 1970 Alb 2 05 O-nitrodiethylanilme-N-ozflde- 3,575,912 4/1971 Albert 260--29.7

4. The process of claim 1, where the active agent is JOSEPH L. SCHOFER,Primary Examiner 5. The process of claim 1, Where the active agent is2,2-(phenylimino)diethanol-N-oxide. 10 W. F HAMROCK, Assistant Examiner6. The process of claim 1, Where the active agent is N methylN-.(2-hydroxy-3-t-butyl-5-methy1benzyl)ani- US. Cl. X.R. line-N-oxide.

7. The process of claim 1, where the active agent is 88.7 B, 93.5 A,83.3, 83.5 N ethyl N-(Z-hydroxy-S-t-amylbenzyl)aniline-N-oxide. 15

